There are two specific theories or reasons Behind the Christmas Celebration on 25th of December:
The first theory is this day was celebrated as some sort of holiday around that time by the people and religions. From Jewish Chanukah to Pagan Winter Solstice to Germanic Yule to Roman Dies Natalis Solis Invicti i.e. birth of the Unconquered Sun, folk celebrates the festival with the trees, decorations, yule logs, mistletoe, and feasts seem to point to a season of celebration. Later Christians added the birth of Jesus as a counter-cultural event and an escape from the pagan holidays for early believers.
Saturnalia Festival of emancipation was celebrated on December 25th which includes gift giving and the triumph of light after the longest night. The Christian sees the reality implicit this pagan tradition that reflects: Christ the Light of the globe, His conquer the night of sin in Luke 1:78-79:
“…Because of the tender mercy of our God, by which the rising sun will come to us from heaven 79 to shine on those living in darkness and in the shadow of death, to guide our feet into the path of peace.”
The second theory is centered around the date “accepted” by the Western Church of March 25 as Annunciation or Immaculate Conception of Jesus in Mary’s womb. December twenty-five is nine months later and therefore celebrated as the birthday of Jesus. In spite of the potential reasons for the date, the church calendar was set within the West throughout Constantine’s reign while the eastern Church control onto the date of January 6 for some time.